Plating method

ABSTRACT

Methods for depositing a metal or metal alloy on a substrate and articles made with the methods are described. The metal or metal alloy is deposited on the substrate electrolytically. The current is periodically interrupted during deposition to improve throwing power and reduce nodule formation on the metal or metal alloy deposit.

BACKGROUND OF THE INVENTION

The present invention is directed to an improved plating method. Morespecifically, the present invention is directed to an improved platingmethod where adjustments of the plating cycle improve throwing power andreduce nodule formation.

Generally, plating substrates with metals includes passing a currentbetween two electrodes in an electrolyte where one of the electrodes isthe substrate to be plated. Electrolytes for depositing a metal on asubstrate typically include one or more metal ions, a soluble salt in asufficient amount to impart conductivity to the electrolyte, andadditives to improve plating uniformity and quality of the metaldeposit. Such additives may include brighteners, levelers, suppressors,antioxidants, and surfactants.

In many conventional plating processes electrodes (cathode and anode) ofan external circuit are immersed into the electrolyte and a DC (directcurrent) is applied across the electrodes. This causes anelectrochemical reaction or reduction resulting in deposition of a metalor metal alloy onto a cathode from metal ions in the electrolyte.Current density profile and primary distribution across the cathodevaries according to the geometric path or distance between the anode andthe cathode leading to deposit thickness variation according to shapeand location of the cathode relative to the anode. This effect is mostapparent when high applied average current densities are used.Therefore, in order to obtain the best uniformity of metal distribution,low applied average current densities are used.

Alternatively, by using PPR (pulse periodic reverse electroplating)current instead of DC current, uniform metal deposits may be produced athigher current densities. This technique is especially useful forelectrolytic copper plating on high aspect printed wiring boards, whichare relatively thick boards with small through-hole diameters. Suchsubstrates present plating problems because of their surface geometry,which affects current distribution, and results in measurabledifferences in current density between the surfaces of the board and thethrough-holes. The current density difference causes uneven metaldeposition with thicker coatings produced on surfaces with highercurrent densities. Generally, board edges and isolated surface circuitryexperience higher current density and result in thicker depositscompared to the center surfaces of the board or the inner surface of thethrough-holes (sometimes referred to as dog-boning). Additionalthickness in these areas may present problems in subsequent processingand assembly operations. A non-uniform surface profile may lead toincreased soldermask being required to meet minimum thicknessrequirements for suitable coverage. A lack of circuit planarity andexcess thickness at through-hole entries may interfere with properlocation of components during assembly, while methods used to reducethis excess thickness may lead to protracted processing times and a lossof production.

PPR current may produce metal deposits with an even thickness on boththe board surface and in the through-holes. A PPR current is created byalternating current modulation between forward and reverse cycles. Thisis accomplished by inverting the current from cathodic to anodic mode,which disrupts the otherwise constant direct current polarizationeffects. The degree of disruption occurs according to the primarycurrent distribution with more in the high current density areas than inthe low current density areas, thus providing a normalization ofdeposition rates across complex geometries at higher applied averagecurrent densities. Moreover, by maintaining thickness uniformity athigher applied average current densities, the overall metal depositionrate is increased and processing times reduced yielding higherproduction output.

Although the use of PPR may result in uniform deposit thickness at highcurrent densities, the surface appearance of the resulting deposit mayrange from a matte to a semi-bright finish relative to the through-holewall, thus producing a non-uniform deposit appearance between high(surface) and low (through-hole) current densities. On the other hand,if DC current is applied, uniformly bright deposits are typicallyproduced throughout the current density range, but low current densitiesare used in order to preserve metal deposit thickness uniformity.Accordingly, neither method provides optimum thickness distribution withuniform metal deposit appearance at high current densities.

Metals that may be plated include, for example, copper, copper alloys,nickel, tin, lead, gold, silver, platinum, palladium, cobalt, chromium,and zinc. Electrolytes for metal plating are used for many industrialapplications. For example, they may be used in the automotive industryas base layers for subsequently applied decorative and corrosionprotective coatings. They also may be used in the electronics industry,such as in the fabrication of printed circuit or wiring boards, and forsemiconductor devices. For circuit fabrication in a printed circuitboard, a metal such as copper is plated over selected portions of thesurface of a printed circuit board and onto the walls of through-holespassing between the surfaces of the circuit board base material. Thewalls of the through-holes are metallized to provide conductivitybetween circuit layers on each surface of the board.

U.S. Pat. No. 6,402,924 discloses a method for depositing a metal onto asubstrate which has apertures or uneven surfaces. The method improvesthe surface appearance including brightness, grain structure andthrough-hole leveling of the deposit while maintaining throwing power athigh current densities. Optimum throwing power is achieved when theplating current density at the center of the through-hole is the same asthat flowing at the substrate surface. Such a current density isdesired, but rarely achieved, to provide for uniform metal layers at thesurface of the substrate and in the through-holes. Circuit defects mayoccur when the current density at the surface of the substrate isdifferent from that of the through-holes.

The method of depositing a metal on the substrate disclosed in the '924patent involves applying a pulsed periodic reverse current across theelectrodes of a plating cell utilizing a peak reverse current densityand a peak forward current density, and varying the ratio of the peakreverse current density to the peak forward current density in periodiccycles to provide a metal deposit of uniform appearance, fine grainstructure and uniform metal thickness onto the substrate. One way tovary this ratio is by holding the peak forward current constant whilevarying the peak reverse current density.

The metal which is deposited onto the substrate depends on theapplication. For example copper is generally used as an undercoat forprotection and conductivity while gold may be used as a topcoat fordecoration, protection and function such as for electrical contacts.Copper and gold alloys also may be plated with this method. Other metalswhich may be deposited by the method include tin, lead, palladium,nickel, silver, zinc, and their alloys. The method is typically used todeposit copper onto printed circuit boards with high aspect ratios,where aspect ratio is board thickness divided by through-hole diameter.

While the method disclosed in U.S. Pat. No. 6,402,924 addresses many ofthe problems discussed above in metal plating, the printed circuit boardindustry continuously seeks greater circuit densification, thusdemanding further improvements in metal plating. To increase density,the industry has resorted to multi-layer circuits with through-holes orinterconnections passing through multiple layers. Multi-layer circuitfabrication results in an overall increase in the thickness of the boardand a concomitant increase in the length of an interconnection passingthrough the board. This means that increased circuit densificationresults in increased aspect ratios and through-hole length and anincrease in the severity of, for example, the dog boning problem. Forhigh density boards, aspect ratios may exceed ten to one.

Another problem encountered in metal plating is the formation ofnodules, also called dendrites, on the metal deposit. Nodules arebelieved to be crystals of the metal being plated and grow out of theplated surface. Although the cause of nodules has been the subject ofsome debate, nodules typically appear when there are incompletesuppressor layers on the substrate. Suppressors generally provide alarge change in the kinetic overpotential of the deposition reaction.This tends to give a more uniform current distribution over the surfaceof the substrate and allows the metal deposition to proceed with aglobal leveling. Suppressors adsorb onto many metals such as copper andare not typically consumed during the metal deposition reaction.Suppressors may be distinguished from levelers, which also increasesurface overpotential but are consumed or altered during metaldeposition. Generally, suppressors are high molecular weight oxygencontaining polymers such as polyethylene oxide, polypropylene oxide,co-polymers (random and block) of the monomers of the precedingpolymers, and other surfactant molecules.

Nodules may range in diameter of from less than 1 micron to as large asseveral millimeters. Nodules are undesirable for a variety ofelectrical, mechanical, and cosmetic reasons. For example, nodules arereadily detached and carried by cooling air flows into electronicassemblies, both within and external to electronic article housings,where they may cause short-circuit failure. Accordingly, the noduleshave to be removed before the plated substrates are assembled intoelectronic articles. Conventional methods of removing the nodulesinvolve laser inspection of each metal plated substrate followed bymanual removal of the nodules by workers using microscopes. Suchconventional methods leave room for worker error and are inefficient.

Accordingly, there is a need for an improved method of depositing metalsand metal alloys on substrates which increase throwing power and reducenodule formation.

SUMMARY OF THE INVENTION

A method including the steps of generating an electric current throughan electrically conductive substrate, electrolyte and anode inelectrical communication; and interrupting the current for one or moreintervals to increase throwing power and to reduce nodules on a metaldeposited on the electrically conductive substrate. An electromotiveforce (emf) or voltage is generated from a suitable source to provide anelectrical current through the electrically conductive substrate,electrolyte and anode which are all in electrical communication witheach other to provide a complete electrical circuit. The electricallyconductive substrate functions as a cathode in the electrical circuit. Ametal or metal alloy is deposited on the electrically conductivesubstrate during current flow. When current flow is interrupted for oneor more intervals, metal deposition stops or is at least reduced.Current interruption for the one or more intervals throughout theplating cycle provides a substrate with metal or metal alloy depositshaving increased throwing power and reduced nodules in contrast to manyconventional metal depositions processes.

The electrolyte contains one or more type of metal ion to provide thesource of the metal or metal alloy deposited onto the electricallyconductive substrate. Generally the source of metal ions is a metal saltwhich is soluble or at least dispersible in the electrolyte diluent. Anymetal which may be plated may be used to practice the method. Inaddition to one or more metal salt, the electrolyte also may include oneor more additives such as brighteners, suppressors, levelers,antioxidants, chelating agents, complexing agents, surfactants, buffers,halogens, and electrically conductive salts. Other additives may beincluded in the electrolyte depending upon the type of metal or metalalloy to be deposited on the substrate.

Any suitable anode may be used to practice the method. Such anodes maybe soluble or insoluble. The anodes may be metal or metal oxides and mayinclude either noble or non-noble metals.

In another embodiment the method includes generating an electric currentthrough an electrically conductive substrate, electrolyte and anode inelectrical communication; and interrupting the current for an intervalof from 0 minutes to 5 minutes within an initial 10 minutes of a metalplating cycle with additional current interruptions of intervals from 0to 5 minutes for every 10 to 20 minutes of the metal plating cycle. Theduration of the metal plating cycle may vary depending upon the metal ormetal alloy thickness desired on the electrically conductive substrate.The method increases the throwing power and reduces nodules on the metalor metal alloy deposit in contrast to many conventional metal and metalalloy plating processes. Additionally, the plated product is resistantto Kirkendall void type defects.

In another embodiment articles made by the methods include one or moresubstrates with one or more metal or metal alloy layers which havethrowing powers of at least 0.5:1. Such substrates have irregulargeometries such as through-holes for the connection of circuit lines,thus the articles may be multi-layered. Accordingly, the methods providefor plating current densities at the center of the through-holes whichare close to or the same as at the surface of the substrate. Thisprovides for metal layers which have uniform thickness or near uniformthickness, thus preventing or reducing circuit defects in the articles.Additionally, the metal and metal alloy layers of the articles havereduced nodules in contrast to many conventional articles and areresistant to Kirkendall void type defects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-B are photographs of a center section and upper section of athrough-hole plated with copper having an average throwing power ofgreater than 0.9:1.

FIGS. 2A-D are photographs of four sections of a surface of a copperplated circuit board showing nodules.

DETAILED DESCRIPTION OF THE INVENTION

As used throughout the specification the following abbreviations havethe following meanings, unless the context clearly indicates otherwise:° C.=degrees Centigrade; mA=milliamps; cm=centimeter; V=volts; hr=hours;min.=minutes; wt %=percent by weight; mm=millimeters; g/L=grams/Liter;mils=0.001 mils/inch; inch=2.54 cm/inch; ms=milliseconds; andSEM=scanning electron micrograph.

The terms “printed wiring board” and “printed circuit board” are usedinterchangeably throughout the specification. “Depositing” and “plating”are used interchangeably throughout the specification. “Multilayer”refers to two or more layers. “Throwing power” is defined as the ratioof the metal deposit thickness at the center of a hole to the metaldeposit thickness at the surface of the hole. “Aspect ratio” meanssubstrate thickness divided by aperture diameter. “Aperture” means holesuch as a through-hole or a depression such as a via in the surface of asubstrate.

All percentages are by weight, unless otherwise noted. All numericalranges are inclusive and combinable in any order, except where it islogical that such numerical ranges are constrained up to 100%.

Methods include the steps of generating an electric current through anelectrically conductive substrate, electrolyte and anode in electricalcommunication; and interrupting the current at one or more intervals inthe plating cycle to increase throwing power and to reduce nodules on ametal deposited on the electrically conductive substrate. Anelectromotive force (emf) or voltage is generated from a suitable powersource to provide an electrical current through the electricallyconductive substrate, electrolyte and anode which are all in electricalcommunication with each other to provide a complete electrical circuit.When current flow is interrupted at the defined intervals, metaldeposition stops or is at least reduced. Current interruption at the oneor more intervals throughout the plating cycle provides a substrate withmetal or metal alloy deposits having increased throwing power andreduced nodules in contrast to many conventional metal depositionprocesses. Additionally, articles plated by the present methods areresistant to Kirkendall void type defects.

Any suitable combination of time intervals may be used throughout theplating cycle to interrupt current as long as the combination ofintervals provides a throwing power of the metal and metal alloysdeposited on the substrate of at least 0.5:1, or such as from 0.5:1 to1:1, or such as from 0.6:1 to 0.95:1, or such as from 0.7:1 to 0.9:1.Such throwing power ranges indicate that plating current densities atthe surface of substrates with irregular geometries are the same orclose to the same as in apertures in the substrates. This provides forglobal leveling of the metal layers which reduces the probability ofcircuit defects in the final articles.

In addition to providing a throwing power of at least 0.5:1, thecombination of time intervals for interrupting the plating cycle reducesthe number of nodules or dendrites formed on the metal and metal alloydeposits. While the cause of nodules has been the subject of somedebate, nodules appear when there are incomplete suppressor layers onthe substrates. The interruptions in the plating cycle appear tocompensate for the incomplete suppressor layers.

In another embodiment the method includes generating an electric currentthrough an electrically conductive substrate, electrolyte and anode inelectrical communication with each other and a source of electromotiveforce or voltage to provide current; and interrupting the current for aninterval of 0 minutes to 5 minutes within an initial 10 minutes of ametal plating cycle with additional current interruptions of 0 to 5minutes for every 10 to 20 minutes of the metal plating cycle.

Intervals of current interruptions within the initial 10 minutes of themetal plating cycle also may range from 5 seconds to 3 minutes, or suchas from 15 seconds to 2 minutes, or such as from 20 seconds to 60seconds. Intervals of current interruption after the initial 10 minutesof the plating cycle also may range from 5 seconds to 3 minutes, or suchas from 15 seconds to 2 minutes, or such as from 20 seconds to 60seconds for every 10 minutes of the remainder of the metal platingcycle, or such as for every 20 minutes of the remainder of the metalplating cycle.

Duration of the metal plating cycle may vary depending upon thethickness of the metal or metal alloy deposit desired on the substrate.Typically, the plating cycles range from at least one minute, or such asfrom one minute to 5 hours, or such as from 30 minutes to 2 hours.

Any suitable plating cycle may be used to practice the methods. Pulseplating is an example of a suitable plating cycle such as periodicreverse pulse plating. DC (direct current) plating also may be used. Netcurrent for the plating cycle is in the cathodic or plating directionsuch that a metal or metal alloy is deposited on the electricallyconductive substrate. Accordingly, the electrically conductive substrateacts as the cathode. Current density is raised from 0 to a desiredcurrent density for depositing the metal or metal alloy on theelectrically conductive substrate. Optimum plating current densitiesvary depending on the metal or metal alloy workers desire to deposit.Such current densities for a given metal or metal alloy are known in theart or may be determined with some experimentation. When a currentinterruption interval is desired during the metal plating cycle, thecurrent is reduced to 0 and the metal plating ceases or is at leastreduced. When the current interruption interval ends, the current isonce again raised to the desired current density for continued metaldeposition.

Any suitable current density may be used to practice the methods. Suchcurrent densities may range 1 mA/cm² and higher, or such from as from 5mA/cm² to 200 mA/cm², or such as from 5 mA/cm² to 125 mA/cm², or such asfrom 5 mA/cm² to 50 mA/cm².

Any suitable anode may be used. The anodes may be soluble anodes such asa copper film or grid. Noble and non-noble insoluble anodes also may beemployed. Examples of such insoluble anodes are iridium dioxide and leaddioxide.

Any suitable electrolyte may be employed to deposit a metal or metalalloy. The composition of the electrolyte may vary depending on the typeof metal or metal alloy to be deposited. In addition to one or moresources of metal ions, the electrolytes also may include one or morediluents, and one or more optional additives such as brighteners,suppressors, levelers, accelerators, antioxidants, buffers, electricallyconductive salts, halides, and surfactants as well as other additives totailor the electrolyte for plating a particular metal or metal alloy.

Examples of metals which may be plated include copper, tin, nickel,cobalt, chromium, cadmium, lead, silver, gold, platinum, palladium,bismuth, indium, rhodium, ruthenium, iridium, zinc, or alloys thereof.Typically the methods are used to deposit copper and copper alloys.Metals are included in the compositions as soluble salts or are at leastdispersible in the electrolyte diluent. Any suitable metal salt orcompound may be employed. Examples of suitable copper compounds includecopper halides, copper sulfates, copper alkane sulfonate, copper alkanolsulfonate, or mixtures thereof. Such copper compounds are water-soluble.

A sufficient amount of a metal salt is included in the electrolyte suchthat the concentration of the respective metal ion is from 0.010 g/L to200 g/L, or such as from 0.5 g/L to 100 g/L. When copper is the metal, asufficient amount of copper salt is employed such that the copper ionconcentration ranges from 0.01 to 100 g/L, or such as from 0.10 g/L to50 g/L.

Any suitable diluents may be used in the electrolyte. Such diluentsinclude water or organic solvents such as alcohol or other suitableorganic solvents. Mixtures of solvents also may be employed.

Sources of halide ions include any suitable chloride salt or othersource of chloride that is soluble in the electrolyte. Examples of suchchloride ion sources are sodium chloride, potassium chloride, hydrogenchloride, or mixtures thereof. Typically, the chloride ion source isincluded in electrolyte such that the chloride ion concentration rangesfrom 0.02 ppm to 125 ppm, or such as from 0.25 ppm to 60 ppm, or such asfrom 5 ppm to 35 ppm.

Brighteners that may be employed in the electrolyte include anybrightener that is suitable for the metal which is to be plated.Brighteners may be specific for the metal or metal alloy. Brightenersmay be included in the electrolyte in amounts of 0.001 ppm to 1.0 ppm.

Examples of suitable brighteners include sulfur containing compoundsthat have a general formula S—R—SO₃, where R is substituted orunsubstituted alkyl or substituted or unsubstituted aryl group. Morespecifically, examples of suitable brighteners include compounds havingstructural formulas HS—R—SO₃X, XO₃—S—R—S—S—R—SO₃X orXO₃—S—Ar—S—S—Ar—SO₃X where R is a substituted or unsubstituted alkylgroup, and preferably is an alkyl group having from 1 to 6 carbon atoms,more preferably is an alkyl group having from 1 to 4 carbon atoms; Ar isan aryl group such as phenyl or naphthyl; and X is a suitable counterion such as sodium or potassium. Specific examples of such compoundsinclude n,n-dimethyl-dithiocarbamic acid-(3-sulfopropyl)ester, carbonicacid-dithio-o-ethylester-sester with 3-mercapto-1-propane sulfonic acid(potassium salt), bissulfopropyl disulfide (BSDS),3-(benzthiazolyl-s-thio)propyl sulfonic acid (sodium salt), pyridiniumpropyl sulfonic sulfobetaine, or mixtures thereof.

Examples of other suitable brighteners include3-(benzthiazoyl-2-thio)-propylsulfonic acid sodium salt,3-mercaptopropane-1-sulfonic acid sodium salt,ethylenedithiodipropylsulfonic acid sodium salt,bis-(p-sulfophenyl)-disulfide disodium salt, bis(ω-sulfobutyl)-disulfidedisodium salt, bis-(ω-sulfohydroxypropyl)-disulfide disodium salt,bis-(ω-sulfopropyl)-disulfide disodium salt, bis-(ω-sulfopropyl)-sulfidedisodium salt, methyl-(ω-sulfopropyl)-disulfide sodium salt,methyl-(ω-sulfopropyl)-trisulfide disodium salt, o-ethyl-dithiocarbonicacid-S-(ω-sulfopropyl)-ester potassium salt, thioglycolic acid,thiosphosphoric acid-o-ethyl-bis-(ω-sulfopropyl)-ester disodium salt,thiophosphoric acid-tris(ω-sulfopropyl)-ester trisodium salt,N,N-dimethyldithiocarbamic acid (3-sulfopropyl) ester sodium salt (DPS),(o-ethyldithiocarbonato)-S-(3-sulfopropyl)-ester potassium salt (OPX),3-[(amino-iminomethyl)-thio]-1-propanesulfonic acid (UPS),3-(2-benthiazolylthio)-1-propanesulfonic acid sodium salt (ZPS), thiolof bissulfopropyl disulfide (MPS), or mixtures thereof.

Examples of suitable levelers include lactam alkoxylates. Examples ofsuch compounds include β-propiolactam ethoxylate,γ-butyrolactam-hexa-ethoxylate, α-valerolactam-octa-ethoxylate,δ-valerolactam-penta-propoxylate, ∈-caprolactam-hexa-ethoxylate, or∈-caprolactam-dodeca-ethoxylate. Such leveling agents are included inthe electrolyte in amounts of from 0.002 to 3 g/L.

Another example of suitable levelers includes polyalkylene glycolethers. Amounts of polyalkylene glycol ether that may be included in theelectrolyte range from 0.005 to 30 g/L. Relative molecular mass may befrom 500 to 3500 g/mole, preferably from 800 to 4000 g/mole.

Examples of such polyalkylene glycol ethers include dimethylpolyethylene glycol ether, dimethyl polypropylene glycol ether,di-tertiary butyl polyethylene glycol ether, stearyl monomethylpolyethylene glycol ether, nonylphenol monomethyl polyethylene glycolether, polyethylene polypropylene dimethyl ether (mixed or blockpolymer), octyl monomethyl polyalkylene ether (mixed or block polymer),dimethyl-bis(polyalkylene glycol)octylene ether (mixed or blockpolymer), and β-naphthol monomethyl polyethylene glycol.

Any suppressor (carrier) that is employed in metal plating may beemployed in the electrolyte. While the concentrations of suppressors mayvary from one electroplating bath to another, suppressors typicallyrange from 100 ppm or greater. Examples of such suppressors arepolyhydroxy compounds such as polyglycols, e.g., poly(ethylene glycol),poly(propylene glycol) and copolymers thereof. The poly(ethylene glycol)may range in molecular weight from 1000 to 12000. Other suitablecompounds include, but are not limited to, polyethylene oxide,polypropylene oxide, and co-polymers (random and block) of the monomersof the polyethylene oxide and polypropylene oxide.

Any suitable buffer or pH adjuster may be employed. Such pH adjustersmay include, for example, inorganic acids such as sulfuric acid,hydrochloric acid, nitric acid, phosphoric acid, or mixtures thereof.Sufficient acid is added to the compositions such that the pH rangesfrom 0 to 14, or such as from 0 to 8, or such as from 0 to 6, or such asfrom 0 to 3. The pH range may vary depending on the metal or metal alloybeing plated.

During plating the electrolyte may range in temperature from 18° C. to110° C., or such as from 25° C. to 60° C. Temperature ranges may varydepending on the metal or metal alloy to be deposited. Copperelectrolytes may be maintained at a temperature range of from 20° C. to80° C. with acid copper baths (pH from 0 to 4) at temperatures of from20° C. to 50° C.

The methods may be used to metal plate any suitable electricallyconductive substrate. When a substrate is made of a dielectric material,the substrate may be treated or activated such that the entire substratemay be made electrically conductive, or it may be selectively activatedsuch that only sections of the substrate are metal plated. Variousmethods are known in the art by which to activate a substrate for metalplating. One such method is to conversion coat the non-conductivesubstrate with metal sulfides such as iron, cobalt, nickel and coppersulfides as disclosed in U.S. Pat. No. 4,810,333. An example of anothermethod of conversion coating is to treat the surface of a non-conductivesubstrate with an acid colloidal solution of a tin-noble metalelectroless metal plating catalyst and then treating the surface with asolution containing dissolved sulfides capable of reacting with themetal plating catalyst to form a sulfide of the catalytic noble metal asdisclosed in U.S. Pat. No. 4,895,739.

Substrates plated with the current interruption methods may be used inany industry where metal plated substrates are used such as in themanufacture of electrical articles. Examples of such electrical articlesinclude printed wiring boards, integrated circuits, electrical contactsurfaces and connectors, electrolytic foil, silicon wafers for microchipapplications, semi-conductors and semi-conductor packaging, lead frames,optoelectronics, and optoelectronic packaging.

For example, in the manufacture of printed wiring boards, metal andmetal alloy deposits with throwing powers of 0.5:1 and greater aredesired. Also metal and metal alloy deposits with reduced nodules aredesired as well to provide an electrical article having reliableperformance. There are various processes for making printed wiringboards, including multi-layer printed wiring boards, known in the art.

In printed circuit board manufacture the substrate typically is an epoxysubstrate filled with glass fibers and is copper clad on at least one ofits surfaces. Through-holes are formed by drilling or punching or anyother suitable method known in the art. The through-holes are thendesmeared to remove any accretions on the walls of the through-holes.Desmearing may be done using sulfuric acid, chromic acid or plasmaetching or etchback of the holes with chromic acid followed by glassetching, or any other suitable method. Following desmearing or etchbackof the through-holes, the board base material is conventionally treatedwith a glass etch that removes glass fibers extending into thethrough-holes from the through-hole walls. This is followed by asolution that cleans the copper surface and conditions the through-holewall to promote catalyst adsorption. Such solutions may be aqueousalkaline surfactants.

The boards may then be immersed in a catalyst pre-dip solution. Suchsolutions include the same medium as the catalyst solution but withoutthe colloid. Proprietary catalyst pre-ip compositions are commerciallyavailable and an example of a suitable material is available from Rohmand Haas Electronic Materials identified as Cataprep™ 404.

The boards are then immersed into an aqueous catalyst composition. Suchcatalyst compositions contain reduction products formed by the reductionof a noble catalytic metal by, for example, tin in an acidic medium. Thereduction product of palladium by tin in the acidic medium is typical.An example of such a catalyst is Cataposit™ R-44 and is available fromRohm and Haas electronic Materials. Conventional non-noble metalcatalysts also may be used. Catalysis may take from 1 minute to 10minutes at a temperature of 20° C. to 70° C.

Optionally, the boards may be treated with an accelerator. A suitableaccelerator removes part of the metal oxide formed by the catalyst suchas tin oxide. Examples of suitable accelerators are hydrochloric acidand perchloric acid. Acceleration is accomplished by immersion of theboards in an aqueous solution of the accelerator for a period of from 1minute to 5 minutes at a temperature of from 20° C. to 70° C.

After application of the catalyst, or accelerator, the boards areconversion coated to make them electrically conductive for metal ormetal alloy deposition. Any suitable method known in the art may be usedto conversion coat the boards. Chalcogenide formation is one processwhich may be used. Chalocogenide formation occurs by contact of thecatalytic layer with a solution of a chalcogen. The chalcogenidetreatment solution may be an aqueous solution of a water solublechalcogen salt. Sulfide is a typical chalcogen. Examples of suitablesulfide salts are alkaline earth metal sulfide salts such as sodium,potassium and lithium sulfides. Chalcogenide treatment solutions havesalt concentrations of 0.1 g/L to 15 g/L.

After conversion coating the boards are immersed into a metal or metalalloy electrolyte (plating bath) for deposition of the metal or metalalloy. Any suitable metal or metal alloy electrolyte may be used todeposit one or more metal layers on the boards. Typically copper or acopper alloy electrolyte is used for plating printed circuit boards.Suitable copper alloys which may be deposited on printed circuit boardsinclude, but are not limited to, copper/tin, copper/bismuth,copper/gold, copper/silver, and copper/nickel. Additional metals whichmay be plated include, but are not limited to, nickel, tin and theiralloys.

A suitable aqueous copper electrolyte includes one or more water solublecopper salts such as copper sulfate pentahydrate in amounts to providecopper ions of 0.1 g/L to 50 g/L, one or more sources of chloride ionsuch as sodium chloride in amounts to provide chloride ions of 5 ppm to35 ppm, one or more brighteners such as BSDS in amounts of 0.1 ppm to0.5 ppm, one or more levelers such as a lactam alkoxylate in amounts of0.005 g/L to 0.2 g/L, one or more suppressors such as poly(ethyleneglycol) with molecular weights of 2500 to 5000 in amounts of 500 ppm to1000 ppm, and one or more inorganic acids such as sulfuric acid insufficient amounts to maintain an electrolyte pH of 0 to 1.

The boards (cathodes) are immersed into the aqueous copper electrolytealong with a counter electrodes (anodes) such as insoluble lead dioxideelectrodes and are connected to a source of an electromotive force suchthat the boards, anodes, electrolyte and emf source are in electricalcommunication with each other to provide a complete electrical circuit.Current density ranges from 10 mA/cm² to 40 mA/cm².

The plating cycle begins by initially raising the current from 0 to alevel suitable for plating and is maintained at that range for 1 minuteto 5 minutes and then is dropped to 0 for 1 minute to 5 minutes tointerrupt the flow of current within the first 10 minutes of the platingcycle. After the first 10 minutes of the plating cycle, the current israised from 0 back to a plating range followed by additional currentinterruptions of from 1 minute to 5 minutes every 10 minutes to 20minutes of the plating cycle until a desired copper layer thickness isachieved on the surface and through-holes of the printed wiring boards.

The copper plating method provides copper metal deposits with a throwingpower of at least 0.5:1, or such as 0.7:1, or such as 0.8:1, or such as0.9:1. Accordingly, the problem of dog-boning found in manyconventionally plated printed wiring boards is reduced. In addition theboards show reduced numbers of nodules in contrast to copper metaldeposits plated with many conventional processes which do not use thecurrent interruption methods.

Plating time for printed wiring boards may range from 45 minutes to 5hours. For circuit board manufacture, a desired metal or metal alloythickness may range from 60 mils to 400 mils, or such as from 80 mils to200 mils, or such as form 90 mils to 150 mils.

The current interruption methods are suitable for metal platingthrough-holes of multi-layer circuit boards with aspect ratios of atleast 5:1, or such as from 7:1 to 10:1. Typically, plated through-holeshave average diameters ranging from 0.02 cm to 0.1 cm, or such as from0.3 cm to 0.7 cm, however, the average diameters of plated though-holesmade by the current interruption methods may vary from the foregoingranges.

Both vertical and horizontal plating processes may be employed. Invertical processes the substrate, such as a printed wiring board, issunk in a vertical position into a container containing an electrolyte.The substrate, which functions as a cathode, is situated in the verticalposition opposite to at least one soluble or insoluble anode. Thesubstrate and the anode are connected to a current source and anelectrical current. Various apparatus for generating an emf are wellknown in the art. The electrolyte is directed continuously through acontainer with the cathode and anode by means of transporting equipmentsuch as a pump. Any suitable pump employed in electroplating processesmay be employed. Such pumps are well known in the electroplatingindustry and are readily available.

In the horizontal plating process, the substrate or cathode istransported through a conveyorized unit in a horizontal position with ahorizontal direction of movement. Electrolyte is injected continuouslyfrom below and/or above and onto the substrate by means of splashnozzles or flood pipes. Anodes are arranged at spacings relative to thesubstrate and are brought into contact with the electrolyte by means ofa suitable device. The substrate is transported by means of rollers orplates. Such horizontal apparatus are well known in the art.

After the printed wiring boards are plated they may undergo furtherprocessing by one or more conventional processes known in the art toform multi-layer circuit boards and then assembled with other componentsto form various electrical articles and devices.

The following examples are provided to better describe the invention,and are not intended to limit the scope of the invention.

Example 1 Throwing Power

A multi-layer copper clad printed circuit board with an array of 0.03cm, 0.07 cm and 0.09 cm through-holes was pre-cleaned by immersing theboard in an aqueous alkaline solution of an organic epoxy solvent ofCircuposit™ Conditioner 3302 (obtainable from Rohm and Haas electronicMaterials, Marlboro, Mass.) at 70° C. mean temperature for 10 minutesand then rinsed with water.

The through-hole walls were then desmeared with a permanganate solutionof MLB Promoter™ 3308 at a mean temperature of 70° C. for 10 minutes andthen rinsed with water. The residues were then neutralized and chargemodification was done using a single treatment solution composed ofhydroxylammonium acetate (50 g/L), methane sulfonic acid (140 g/L),surfactants (5 g/L), polyelectrolyte (10 g/L) and one liter of water for5 minutes at 50° C. The board was then rinsed with water.

A conductive particle coating was then applied to the board. Theconductive particle coating was a basic carbonaceous dispersion ofgraphite. The board was submerged in the dispersion for 5 minutes atroom temperature. The board with the conductive particle coating wasthen placed in an air circulating oven maintained at 80° C. until thecoating was dry.

The copper cladding on the board was microetched to remove thedispersion coating from the copper cladding. The board was then sprayedwith water. The non-conductive portions of the board were then ready formetallizing.

The board was then placed into a plating tank with an aqueouselectrolyte including 10 g/L of copper sulfate pentahydrate, 150 g/L ofsulfuric acid, 50 ppm of chloride ions, 200 ppm of poly(ethyleneglycol), and 0.5 ppm of BSDS. The pH of the electrolyte was maintainedfrom 0 to 1 throughout the plating cycle.

The board was joined to an emf source along with a copper anode toprovide a complete circuit. During plating the current was maintained at50 mA/cm². The plating cycle began by raising the current from 0 to 50mA/cm² and maintaining the current at 50 mA/cm² for 5 minutes to platecopper on the board followed by decreasing the current down to 0 for a 1minute current interruption interval and then increasing the currentback to 50 mA/cm² for the remaining initial 10 minutes of the platingcycle. After the initial 10 minutes of the plating cycle was completed,the current was decreased to 0 for 1 minute current interruptionintervals every 20 minutes until the plating cycle was completed.

FIGS. 1A and 1B show a SEM of a cross-section of one of the copperplated through-holes. FIG. 1A shows the center section of thethrough-hole and 1B shows the upper or surface portion of thethrough-hole. The throwing power of the copper metal layer of thisthrough-hole was determined to be greater than 0.9 (diameter of thethickness of the copper layer at the center of the through-hole/thethickness of the copper layer at the surface of the through-hole).Optimum throwing power is 1. Accordingly, the current interruptionmethod provided a copper metal layer having good throwing power.

Example 2 Comparative Dendrite Reduction

Two multi-layer copper clad printed circuit boards with an array of 0.03cm, 0.07 cm, and 0.09 cm through-holes were pre-cleaned and conditioncoated by the same method as described in Example 1. After thepre-treatment process was completed each board was placed into an copperelectrolyte for copper metal deposition.

The copper electrolyte included 20 g/L of copper sulfate pentahydrate,250 g/L of sulfuric acid, 100 ppm of chloride ions, 100 ppm ofpoly(ethylene glycol), and 0.5 ppm of BSDS. The pH of the electrolytewas maintained from 0 to 1 throughout the plating cycle.

Each board was placed in a separate plating tank. One board acted as acontrol where it was plated using a conventional plating process whilethe other was plated using a current interruption method. The currentdensity for each was maintained at 50 mA/cm² during copper plating. Eachmulti-layer board was connected to an emf source along with a copperanode as the counter electrode. The plating cycle lasted for 2 hours.

After the plating cycle was over the control board was analyzed forstringy nodules or dendrites using a conventional laser inspectiontechnique. Dendrites were detected on the surface of the copper platedboard. FIGS. 2A and 2B are SEMs of the board showing large dendrites onthe surface of the board across the copper plated portion of thesurface.

The second board was copper plated with a current interruption method.The electrical potential was initially raised from 0 to 50 mA/cm² andmaintained at 50 mA/cm² for 2 minutes then the current was interruptedfor 5 minutes by reducing the current to 0. The current was then resumedby raising it back to 50 mA/cm² for the remainder of the first 10minutes of the plating cycle. After the initial 10 minutes of theplating cycle was completed, the current was interrupted for 2 minutesfor every 10 minutes of the plating cycle until the plating cycle wascompleted. The cycle was completed after 2 hours.

After the plating cycle was completed the board was analyzed fordendrites. Analyses were performed using the conventional laserinspection method as for the control board. Some dendrites were detectedbut fewer were detected than in the control board. FIGS. 2C and 2D showSEMs of a portion of the copper plated board. A single small stringydendrite is shown at the plated copper at the edge of a through-hole inboth FIGS. 2C and 2D. The current interruption method reduced the numberof dendrites in contrast to the control board which was plated by aconventional process. Accordingly the current interruption method was animprovement in plating copper metal.

Example 3 Comparative Dendrite Reduction on Double Sided Panels andThrowing Power

48 FR-4/epoxy copper clad panels were provided. Holes were drilled ineach panel to provide an array of through-holes having diameters of 0.03cm, 0.07 cm, and 0.09 cm. Each panel was pre-treated to removeaccretions from the though-holes using conventional permanganatesolutions. Selective portions of the non-conductive parts of each boardwere conversion coated using a conventional chalcogen solution andconventional processes to make the portions electrically conductive forcopper metal deposition.

Each panel was plated in an aqueous copper electrolyte composed of 50g/L of copper sulfate pentahydrate, 250 g/L of sulfuric acid, 100 ppm ofchloride ions, 300 ppm of poly(ethylene glycol) and 1 ppm of BSDS. ThepH of the electrolyte was maintained from 0 to 1 during the platingcycle.

All of the panels were joined to an emf source along with a copper anodeto provide a complete electrical circuit. The current during copperplating was maintained at 50 mA/cm². 12 panels (controls) were copperplated using a conventional plating process, while the remaining 36panels were copper plated with a current interruption method.

The conventional plating cycle continued for 1 hour. Each panel was thenanalyzed for dendrite formation using the conventional laser inspectionmethod. All of the panels showed dendrite formation. The number ofdendrites was counted by workers using microscopes. The results areshown in the table below.

24 panels were plated by initially raising the current from 0 to 50mA/cm² with a current interruption for 5 minutes followed by raising thecurrent from 0 to 50 mA/cm² for the remainder of the initial 10 minutesof the plating cycle. Current interruptions of 1 minute intervals werethen done for every 10 minutes of the plating cycle until the platingcycle was completed after 1 hour. Each board was then analyzed using theconventional laser inspection method. Workers used microscopes tomanually count the number of dendrites on each board. The results areshown in the table below.

12 panels were plated by initially raising the current from 0 to 50mA/cm² with a current interruption for 5 minutes followed by raising thecurrent from 0 back to 50 mA/cm² for the remainder of the initial 10minutes of the plating cycle. Current interruptions of 1 minuteintervals were then done every 20 minutes for the remainder of theplating cycle. The plating cycle lasted 1 hour. Each panel was thenanalyzed using the conventional laser inspection method. Workers usedmicroscopes to manually count the number of dendrites on each board. Theresults are shown in the table below.

TABLE TOATL PANELS NUMBER AVERAGE USED OF DENDRITES NUMBER/PANEL CONTROL12 14 1.2 10 MINUTE 24 6 0.25 INTERRRUPTION 20 MINUTE 12 2 0.17INTERRUPTION

The results showed that the current interruption methods reduced thenumber of dendrites formed during copper plating in contrast to theconventional process of copper plating. The control panels had anaverage number of dendrites of 1.2 per panel, while the panels platedwith the current interruption methods had averages of 0.25 and 0.17 perpanel.

In addition to having reduced dendrite formation, the currentinterruption methods also had an average throwing power increase ofgreater than 90% over the throwing power of the panels plated using theconventional process. Accordingly, the current interruption methodsprovide for an improved copper plating method.

What is claimed is:
 1. A method comprising generating an electriccurrent through an electrically conductive substrate, electrolyte andanode in electrical communication; and interrupting the current bybringing current density to 0 for an interval of 1 minute within theinitial 10 minutes of the copper metal plating cycle with additionalinterruptions of intervals of 1 minute for every 20 minutes of thecopper metal plating cycle, wherein the electric current is directcurrent.
 2. A method comprising generating an electric current throughan electrically conductive substrate, electrolyte and anode inelectrical communication; and interrupting the current by bringingcurrent density to 0 for an interval of 5 minutes within the initial 10minutes of the copper metal plating cycle with additional interruptionsof intervals of 2 minutes for every 10 minutes of the copper metalplating cycle, wherein the electric current is direct current.
 3. Themethod of claim 2, wherein the substrate is a printed wiring board,integrated circuits, electrical contact surfaces, connectors,electrolytic foil, silicon wafers, semi-conductors, lead frames oroptoelectronics.